JP2017218670A - Metastable austenitic stainless steel band or steel sheet and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a metastable austenitic stainless steel band or steel sheet having high strength and high ductility and a manufacturing method therefor.SOLUTION: There are provided a metastable austenitic stainless steel band or steel sheet containing, by mass%, C:0.05 to 0.15%, Si:0.05 to 1%, Mn:2% or less, Cr:16 to 18%, Ni:4 to 11%, Mo:2.5% to 3.5%, one or two kind selected from a group of Al:0.1% to 3.5% and Ti of 0.1% to 3.5% and the balance Fe with inevitable impurities, having 2-phase structure of α' phase and γ phase, where the γ phase is constituted by γphase and γphase, and having total of the γphase and the γphase of 15 to 50 vol.%, γphase area ratio defined by the formula 2 (=100×(total area percentage of the γphase in whole observed area) of 1% to 20%, 0.2% yield strength (YS) of 1400 N/mmto 1900 N/mmand "YS×EL balance"(YS EL) satisfying 21000 to 48000, and a manufacturing method therefor.SELECTED DRAWING: None

Description

  The present invention relates to a metastable austenitic stainless steel strip or steel plate having an excellent balance between strength and ductility, and a method for producing the same.

  Functional parts of precision equipment such as smartphones, notebook computers, cameras, etc., and highly durable skeletal structural parts such as automobiles and aircraft, etc., are reduced in thickness and weight by increasing strength while satisfying requirements for workability and dimensional accuracy. Figured. Furthermore, since the load at the time of component driving increases due to the reduction in size and weight of the device, excellent durability such as strength that can withstand severe use and repeated fatigue strength is required.

In particular, in the case of skeletal structural parts for automobiles, development relating to high strength and high ductility has been energetically performed. For example, a development example of γ-SUS or TWIP (Twinning Induced Plasticity) steel having a strength-ductility balance of conventional TRIP (Transformation Induced Plasticity) steel to which Mn or Ni exceeding 20 mass% is added is known. However, these steels not only increase the component cost, but also have a problem that cold rolling becomes difficult when manufactured as a steel strip or steel plate. In many cases, since no Cr is added, rust prevention treatment is inevitable.
Today, the low-alloy TRIP-type composite structure steel that is regarded as the most important material has only obtained TS: 980MPa-EL: 30% and TS: 1180MPa-EL: 25% (see Non-Patent Document 1). A technology relating to steel strips and steel sheets having a yield strength (YP) ≧ 1400 MPa required as a material and high ductility has not been developed yet.

For example, in Patent Document 1 (Japanese Patent Laid-Open No. 2002-173742), a solution-induced martensite phase (α ′ phase) is formed by cold rolling after solution treatment of a stainless steel strip for the purpose of improving shape flatness. Next, by heating at 500 ° C. to 700 ° C. and performing reverse transformation treatment to generate 3% by volume or more of γ _T phase (reverse transformation austenite phase) in the α ′ phase, the shape flatness having a Vickers hardness of 400 or more is obtained. It is said that an excellent high-strength austenitic stainless steel strip can be manufactured.
However, the amount of γ _T phase is highly temperature dependent, and depending on the chemical composition, the amount of γ _T phase exceeds 60% when subjected to reverse transformation treatment at a temperature of 500 ° C. or higher, and the strength is 1400 N / mm ² or more. Hard to get. Moreover, although improvement in ductility is manifested by holding for a short time (for example, 1 to 5 minutes), it is an unstable processing condition in which a decrease in ductility proceeds rapidly in a holding time of about 5 to 15 minutes, and stable mechanical properties. It is difficult to provide a steel strip or steel plate with Further, since precipitation of carbides such as Cr—C and Mo—C does not proceed, the 0.2% yield strength is slightly increased as compared with the present invention. Therefore, it is impossible to achieve both high strength and high ductility, which are the objects of the present invention.

Patent Document 2: Japanese Patent Laid-Open No. 54-120223 discloses a stainless steel plate having the same component system as the stainless steel strip or steel plate according to the present invention, which includes a solution treatment, cold rolling of 20 to 80%, It is disclosed to perform low temperature tempering at 400 ° C. However, in Patent Document 2, 2.0% or less (only 1.15% of Example 9 in the specification) is added as an effective component for improving the corrosion resistance of Mo, and Mo is added as a precipitation strengthening component in low-temperature heat treatment. Not done. In addition, with such a small amount of added Mo, it is difficult to exhibit the “function of precipitation strengthening in low-temperature heat treatment”.
Patent Document 3: Japanese Patent Application Laid-Open No. 2012-201924 discloses that a stainless steel plate is annealed at 700-1100 ° C., cold-rolled at 10% or more, and an aging treatment at 300 ° C. However, this stainless steel plate does not contain Mo, and cannot exhibit the “precipitation strengthening function in low-temperature heat treatment” by adding Mo.

In Non-Patent Document 2, the balance of tensile strength (TS) and elongation (EL) in the range of 300 ° C. to 500 ° C. is used as an index. The tensile strength (TS) increases to about 1750 N / mm ² , but 0 The 2% proof stress is not only about 1250 N / mm ² , but also Fe—Cr—C based steel having a γ phase as a parent phase, which is out of the category of metastable austenitic stainless steel to which the present invention belongs.

For general-purpose stainless steel containing 12 mass% or more of Cr, it is a metastable austenitic stainless steel represented by SUS304, or austenite (γ phase) by cold working by reducing the Ni content particularly when strength is required. For example, SUS301, which can be transformed into a martensite (α ′ phase) by machining-induced transformation, is used. Although these stainless steels have advantages when focusing on individual properties such as strength and workability, when trying to obtain 0.2% proof stress (YS) exceeding 1400 N / mm ² , the elongation (EL) is It becomes 10% or less, and the YS-EL balance (indexed by YS × EL) is about 14000. Therefore, not only does the material have a sufficient balance between strength and ductility, but the reliability as a component is not sufficient.

There is SUS631 precipitation hardening stainless steel as a steel type using precipitation strengthening by Ni ₃ Al by adding about 1% Al based on the chemical component of SUS301 for the purpose of increasing the strength after forming a part. In this case, a precipitation hardening heat treatment is necessary after the molding process, which not only increases the cost at the secondary processing manufacturer, but also causes deformation and dimensional variations of the molded part due to the heat treatment. In addition, the ductility of the component itself decreases due to precipitation hardening, so that the toughness of the component itself decreases. Therefore, there is a universal need for a material having a good balance between strength and ductility that does not require post-treatment that causes dimensional changes such as heat treatment after molding.

JP 2002-173742 A JP 54-120223 A JP 2012-201924 JP

“Iron and Steel” Vol.100 (2014) No.1, P.82-93 Nanoscale austenite reversion through partitioning, segregation and kinetic freezing: Example of a ductile 2 GPa Fe-Cr-C steel L. Yuan el al.l Acia Malerialia 60 (2012), p.2790-2804

As a result of intensive studies to solve the above-mentioned problems, the present inventors pay attention to the potential of the α ′ phase generated by processing-induced transformation, and the 0.2% proof stress (YS) is high strength up to about 1400 N / mm ^2. We worked on the development of metastable austenitic stainless steels that can be made into steel.
Conventional metastable austenitic stainless steel can achieve both ductility before processing and strength after processing by processing-induced transformation and aging precipitation strengthening by cold working, but the cost and dimensional change of aging precipitation strengthening, etc. Is a problem. Especially for electronic parts and precision parts that require high precision, the dimensional change has a great influence on the performance of the final product, so heat treatment after molding the parts requires advanced technology and know-how.

Therefore, the present inventors have accumulated in the α ′ phase by performing a low temperature heat treatment at 250 to 480 ° C. after transforming the metal structure of this stainless steel into α ′ by cold working of 1 to 80%. By diffusing and concentrating supersaturated solute carbon into a γ phase with a volume ratio of several percent using the strain energy as a driving force, the adjacent α ′ phase can be reversely transformed into a γ _T phase using the γ phase as a nucleus. As a result of the heat treatment, carbides of Cr and Mo are finely precipitated in the α ′ phase, so that the strength is further increased and simultaneously the processing induced transformation (TRIP) effect by dispersing the γ _T phase is 1400 N / mm. It has been found that 0.2% proof stress (YS) of ² or more and elongation (EL) of 15% or more can be realized, and the present invention has been completed. Moreover, under suitable conditions within the scope of the present invention, it is possible to achieve both 0.2% proof stress (YS) of 1550 N / mm ² or more and elongation (EL) of 23% or more. YS-EL represented by the following formula 1 Realized more than 35,000 characteristics in balance.
“YS-EL balance” = YS · EL Equation 1
The α ′ phase indicates a processing-induced martensite phase.
The γ _R phase indicates the retained austenite phase.
The γ _T phase indicates a reverse transformation austenite phase.
The present invention is a steel strip or steel plate that has all the characteristics of high strength, high ductility, and high corrosion resistance, and a method for producing the same.

The present invention has been made on the basis of the above-described knowledge, and the metastable austenitic stainless steel strip or steel plate according to the present invention is in mass%, C: 0.05 to 0.15%, Si: 0.05. -1%, Mn: 2% or less, Cr: 16-18%, Ni: 4-11%, Mo: 2.5% -3.5%, and Al: 0.1% -3.5% and It contains one or two selected from the group of 0.1% to 3.5% Ti, the balance consists of Fe and inevitable impurities, and is a two-phase structure of α ′ phase and γ phase, and γ phase is γ _T Phase and γ _R phase, the total of γ _T phase and γ _R phase is 15 to 50% by volume, and the area ratio of γ _T phase defined in Formula 2 is 1% or more and 20% or less, 0 .2% yield strength (YS) is ^{1400N / mm 2 ~1900N / mm 2} , shown in formula 1 "YS-EL balance" is at least 21000-48000 It has the characteristics to satisfy | fill.
In addition, the method for producing a metastable austenitic stainless steel strip or steel sheet according to the present invention is performed by cold-working a stainless steel strip or steel sheet having this composition, from an austenite phase (γ phase) to a work-induced martensite phase (α ′ Phase) and a low temperature heat treatment in a range of 250 ° C. to 480 ° C. on the stainless steel strip or steel plate on which the work induced martensite phase (α ′ phase) is formed, and the work induced martensite phase forming step. And a step of growing an austenite phase (γ _T phase) from the martensite phase (α ′ phase) formed in step (b).
“YS-EL balance” = YS · EL Equation 1
γ _T- phase area ratio (%) = 100 × (total area ratio of γ _T- phase in the entire observation area) Equation 2
However, the α ′ phase is a work-induced martensite phase, the γ phase is a phase in which the γ _T phase and the γ _R phase are combined, and the γ _T phase is a reverse transformed austenite phase having an area per particle of 5 μm ² or more and 20 μm ² or less, The γ _R phase represents an austenite phase other than the γ _T phase, YS represents 0.2% yield strength, and EL represents elongation.
Such characteristics satisfy the TRIP of the γ _T phase dispersed in the α ′ phase by satisfying 0.2% proof stress (YS) exceeding 1400 N / mm ² by the α ′ phase hardened by precipitation of carbides such as Cr or Mo. The inventor estimates that an elongation (EL) of more than 15% is manifested by the effect.

  Hereinafter, the metastable austenitic stainless steel strip or steel plate according to the present invention will be described.

(About composition)
The stainless steel strip or steel plate according to the present invention is in mass%, C: 0.05 to 0.15%, Si: 0.05 to 1%, Mn: 2% or less, Cr: 16 to 18%, Ni: One or two selected from the group consisting of 4 to 11%, Mo: 2.5% to 3.5%, and Al: 0.1% to 3.5% and Ti 0.1% to 3.5% Is a metastable austenitic stainless steel.

  C is added in an amount of 0.05% or more in order to impart the necessary strength to the processing-induced transformation during cold rolling and the α ′ phase after transformation. However, if added over 0.15%, the austenite phase is stabilized, so that it is difficult to develop work-induced transformation during cold rolling, and at the same time, the upper limit is made 0.15% to deteriorate secondary workability such as punching. It was as follows.

  Since Si is an important element in steelmaking as a deoxidizing material, 0.05% or more is added. However, if added over 1%, the rollability and toughness are lowered, so the upper limit was made 1%.

  Mn is an element that stabilizes the austenite phase together with Ni, and if added in a large amount, a structure having a processing-induced α ′ phase of 50% or more cannot be obtained by ordinary cold rolling. Therefore, in the present invention, the upper limit is defined as 2%. The lower limit is not particularly specified, but it is preferably 0.1% as a countermeasure against hot cracking during hot rolling.

  Cr is added in an amount of 16% or more in order to impart corrosion resistance as stainless steel. However, if added over 18%, the austenite phase is stabilized, so that a sufficient amount of the processing-induced transformation α ′ phase cannot appear in the normal cold rolling process. Therefore, in the present invention, the upper limit is limited to 18%.

  Ni is an austenite stabilizing element, and it is essential to add a predetermined amount in order to maintain the structure before cold rolling in a metastable austenite state. In the present invention, 4% or more is added as a lower limit for obtaining a metastable austenite phase after solution treatment. However, if added over 11%, the austenite phase becomes stable, so that a structure composed of a processing-induced transformation α ′ phase of 50% or more in volume ratio after ordinary cold rolling cannot be obtained. Therefore, the upper limit was limited to 11%.

  Mo is an important element in the present invention. Although Mo is known to be an effective element for improving the pitting corrosion resistance of stainless steel, it is also an important precipitation strengthening element in the low-temperature heat treatment in the present invention. In the present invention, 2.5% or more is specified as the lower limit value for obtaining precipitation strengthening of the α ′ phase by Mo carbide, and when the Mo addition amount is increased, not only the precipitation strengthening ability is saturated but also the alloy cost is increased. Since this is disadvantageous, the upper limit is specified as 3.5%.

  In addition, one or more elements selected from elements such as Ti and Al are added for the purpose of precipitation strengthening. These individual element addition amounts depend on the balance with other elements, but are generally 0.1% to 3.5%. Further, in order to improve the corrosion resistance of the α ′ phase after the processing-induced transformation, Cu: 0.4 to 1.0% can be blended in mass%. If it is less than 0.4%, the remarkable effect of improving corrosion resistance is not recognized. Conversely, if it exceeds 1.0%, problems in the manufacturing process such as hot cracking during hot rolling tend to occur.

  The steel strip or steel plate of the present invention contains P, N, S, O, etc. as unavoidable impurities, but the amount of the impurities is in a range that is included in a normal manufacturing process, which hinders the object of the present invention. It is acceptable because there is nothing.

(About metal structure)
Metastable austenitic stainless steel strip or steel sheet according to the present invention, a two-phase structure of α'-phase and gamma-phase, gamma-phase is composed of a gamma _T phase and the gamma _R-phase, and gamma _T-phase and the gamma _R phase Is a total volume of 15 to 50% by volume (α ′ phase is 50 to 85% by volume), and the γ _T phase area ratio defined in Equation 2 (= 100 × (total area ratio of γ _T phase in the entire observation area)) Is 1% or more and 20% or less.
Here, if the total of the γ _T phase and the γ _R phase is less than 15% by volume (the α ′ phase exceeds 85% by volume), the γ phase is insufficient, the TRIP effect is lost, and the elongation is lowered.
Conversely, if the total of the γ _T phase and the γ _R phase exceeds 50% by volume (the α ′ phase is less than 50% by volume), the γ phase becomes excessive, the TRIP effect is lost, and the strength is lowered.
If the γ _T phase area ratio is less than 1%, the γ phase is insufficient, the TRIP effect disappears, and the elongation decreases.
If the γ _T phase area ratio exceeds 50%, the γ phase becomes excessive, the TRIP effect is lost, and the strength is lowered.

(About characteristics)
Such a metastable austenitic stainless steel strip or steel sheet having a composition and a metal structure has a 0.2% proof stress (YS) of 1400 N / mm ^{2 to} 1900 N / mm ² , preferably 1550 N / mm ^{2 to} 1900 N / It is possible to have a characteristic of satisfying “YS-EL balance” (= YS · EL) of at least 21000-48000, preferably 35000-48000 at mm ² .

(About manufacturing method)
The stainless steel strip or steel plate having the above composition is cold worked to form a work-induced martensite phase (α ′ phase) from the austenite phase (γ phase), and then the stainless steel strip or steel plate is heated to 250 ° C. to 480 ° C. Austenite phase (γ _T phase) is grown from the martensite phase (α ′ phase) formed in the processing-induced martensite phase formation step by performing low-temperature heat treatment in a range, thereby having the above-described metal structure and characteristics A metastable austenitic stainless steel strip or steel plate can be obtained.

The inventor presumes that the metastable austenitic stainless steel strip or steel sheet according to the present invention has the above-mentioned characteristics by the following mechanism. That is, by performing low-temperature heat treatment in such a metallographic state, the supersaturated solid phase in the α ′ phase is driven by the strain energy accumulated in the α ′ phase that has undergone work-induced transformation from the γ phase during cold working. Molten C diffuses and concentrates into a fine γ _R phase that becomes the nucleus of reverse transformation, and the growth of the γ phase proceeds. Furthermore, by maintaining the temperature at a predetermined temperature, the precipitation hardening phenomenon of the α ′ phase proceeds. By controlling these phenomena with various parameters, it is considered that both the strength of the α ′ phase and the high ductility due to the processing-induced transformation of the γ phase can be achieved. In other words, the “YS-EL balance” in the formula 1 can be made to satisfy 21000 or more.
When the ratio of the α ′ phase after cold working is less than 50%, the strain energy accumulated in the α ′ phase is low, so that no diffusion or concentration of C occurs from the α ′ phase to the γ phase. For this reason, the properties of the present invention are not manifested, and since the cold working rate is low and the dislocation density in the α ′ phase is low, the balance between strength and elongation “YS-EL balance” does not exceed the properties of conventional materials. Absent.

“YS-EL balance” = YS · EL Equation 1

(About volume ratio)
Next, the martensite phase (α ′ phase) and austenite phase (γ phase) in the present invention are evaluated by using an EBSD (Backward Electron Scattering Diffraction) method to obtain a plane (so-called RD plane) perpendicular to the rolling direction of the steel material. Calculated based on Phase measurement results when an observation area of 0.05 mm × 0.05 mm or more and the number of crystal grains included is at least 1000 or more and an orientation difference of 5 ° or more is defined as a grain boundary. It is assumed that the obtained area ratio is read as the volume ratio of the present invention. The same applies to volume%.

(Characteristic)
The stainless steel strip or steel sheet having the composition and metal structure according to the present invention is characterized by 0.2% proof stress (YS) of 1400 N / mm ² or more and elongation (EL) of 15% or more. By satisfying these, the “YS-EL balance” is at least 21,000 or more. Moreover, in the preferred conditions within the scope of the present invention, a 0.2% proof stress (YS) of 1550 N / mm ² or more and an elongation (EL) of 23% or more are compatible, and the “YS-EL balance” exceeds 35,000 characteristics. Can be realized. These are the characteristics which had the outstanding intensity | strength and ductility which were not obtained with the stainless steel strip or steel plate until now.

(Manufacturing method)
An example of a manufacturing method for obtaining the above-described metal structure and characteristics according to the present invention will be described below in comparison with a conventional method for manufacturing a stainless steel strip.
First, a conventional method for producing a conventional stainless steel strip or steel plate will be briefly described, and then an example of a method for producing a stainless steel strip or steel plate according to the present invention will be described.

Conventionally, a precipitation strengthened metastable austenitic stainless steel strip (e.g., SUS631 (17-7PH)), which has been conventionally used, is produced by a conventional method using a stainless steel strip with an increased skin pass obtained by conventional means ( For example, after rolling according to a reduction ratio of 85%, solution heat treatment is performed. In this solution heat treatment, for example, the solution is made into a solid solution at 1100 ° C. and then cooled with water. Next, the martensite transformation treatment, specifically, rolling at a rolling reduction of 60%, for example. Thereafter, in order to utilize precipitation strengthening of the intermetallic compound, for example, precipitation hardening is performed at 475 ° C. By such treatment, a stainless steel strip having a 0.2% proof stress (YS) of about 1400 N / mm ² is obtained, but the elongation is a low value of about 1 to 10%. This is not intended for reverse transformation. When the reverse transformation treatment is performed at a temperature equal to or higher than the precipitation hardening temperature, for example, 500 ° C. or higher, an increase in elongation can be expected, but not only the reverse transformation but also the solid solution of the precipitated intermetallic compound proceeds to 0.2. The% yield strength (YS) decreases. Therefore, 0.2% yield strength (YS) of 1400 N / mm ² or more cannot be expressed.

Below, a suitable example of the manufacturing method for obtaining the stainless steel strip or steel plate which concerns on this invention is demonstrated.
First step: In this step, a stainless steel strip (eg, SUS631 (17-7PH)) having the composition of the present invention obtained by conventional means is cold-rolled. This rolling process is intended to increase the ratio of the α ′ phase by processing-induced transformation. For this reason, the cold work rate varies depending on the steel strip composition, sheet thickness, etc., but the cold work rate is in the range of 20% to 90%, preferably 30% or more.

Second step: Next, a solid solution heat treatment is performed on the rolled stainless steel strip. In this heat treatment, the α ′ phase transformed by cold working is reversely transformed into a γ _T phase, and supersaturated C in the α ′ phase is uniformly dispersed in the γ phase, and then the martensitic transformation is performed. This is intended to make the metal structure uniform in processing. The heat treatment temperature for solid solution varies depending on the composition of the stainless steel strip, but is, for example, in the range of 900 ° C. to 1150 ° C., preferably 1000 ° C. or higher. Then, it is rapidly cooled (for example, water cooled) after heating.

Third step: Next, martensite transformation treatment is performed. The rolling reduction in this treatment varies depending on the desired properties, steel strip composition, sheet thickness, etc., but is in the range of 0% to 60%, preferably in the range of 5% to 40% with respect to the steel material or steel strip before processing. It is.
When the rolling reduction exceeds 60%, the γ phase that becomes the core of reverse transformation is insufficient, and the structure within the scope of the invention cannot be obtained by the subsequent reverse transformation treatment.

  Fourth step: In the third step, the steel strip or the steel plate subjected to the martensitic transformation process according to the desired characteristics is subjected to low temperature heat treatment in the range of 250 ° C to 480 ° C, preferably in the range of 300 ° C to 450 ° C. When the temperature is less than 250 ° C., the diffusion and concentration of the supersaturated solid solution C in the α ′ phase does not sufficiently occur, and the γ phase does not grow, so that the improvement of the strength ductility balance cannot be expected. Further, since the temperature near 480 ° C. is close to the solution start temperature, diffusion of supersaturated solid solution C in the α ′ phase is promoted, and the above TRIP effect does not occur because the stable γ phase grows excessively. In addition, the ductility is lowered and the strength is also lowered. On the other hand, the steel strip or steel plate having the composition of the present invention that has undergone these steps improves the balance of strength (YS) and elongation (EL) by changing the ratio of α ′ phase and γ phase, The characteristics of the present invention can be obtained.

In addition, when PH stainless steel is subjected to reverse transformation heat treatment in the vicinity of a precipitation hardening temperature (for example, 500 ° C.) normally used for the purpose of precipitation of intermetallic compounds, intermetallic compounds are precipitated. As a result, the strength is increased, but the ductility is significantly reduced. Therefore, even if the intermetallic compound is within the range of the processing conditions of the present invention for the precipitated PH stainless steel or the like, the temperature is lower than that of the metastable austenitic stainless steel other than the PH stainless steel described above (for example, 250 ° C to 300 ° C). It was found that both high strength and high ductility can be achieved by heat treatment at ℃) and utilizing the increase in γ _T phase and carbide precipitation.
Furthermore, when the precipitation hardening heat treatment is performed at a temperature usually performed after forming the target shape (for example, 500 ° C.), the diffusion of solute atoms is promoted, so that the precipitation of intermetallic compounds is accelerated. It was found that an increase in strength could be expected.

  In view of the above circumstances, the present inventor has focused on the above-described PH stainless steel typified by SUS631 as a metastable austenitic stainless steel strip or steel plate excellent in balance between strength and ductility.

  By satisfying the conditions as shown in these first to fourth steps, it is possible to realize a metastable austenitic stainless steel strip or steel plate having a characteristic that the YS-EL balance exceeds at least 21,000.

According to this manufacturing method, characteristics that were previously impossible to achieve can be achieved without greatly deviating from the scope of the secondary processing step that is normally performed and without significantly increasing the manufacturing cost and environmental load. Stainless steel strips or steel plates can be manufactured. In addition, the manufacturing process shown in the first process or the second process may be repeatedly performed according to the state of the raw material, and then the martensite transformation process shown in the third process may be performed.
In addition, the manufacturing method of the stainless steel strip or steel plate which concerns on this invention mentioned above is an example to the last, Comprising: This invention is not limited to this manufacturing method.

According to the present invention, the strength that is a characteristic of metastable austenitic stainless steel and the ductility that is a characteristic of a high-formability steel sheet can be achieved at a high level.
As a result, it is possible to apply to parts requiring extremely high strength in structure and to design parts with more complicated shapes, which could not be realized with conventional high-strength materials.
The base metastable austenitic stainless steel strip has a high Cr and Ni content and is superior in corrosion resistance compared to high strength and high ductility materials such as automotive steel plates. There are cases in which surface treatment for the purpose of the treatment becomes unnecessary, and not only strength and ductility but also application to applications that require corrosion resistance can be expected.
In a conventionally known metastable austenitic stainless steel strip, 0.2% yield strength (YS) increases with an increase in the cold work rate, but elongation (EL) decreases. As a result, not only is the workability inferior, but in a precipitation hardening type material, dimensional changes due to heat treatment after processing are inevitable.
In contrast, in the present invention, not only a high 0.2% proof stress (YS) exceeding 1400 N / mm ² can be obtained, but also an elongation (EL) exceeding 15% can be simultaneously achieved.

FIG. 1 is a drawing-substituting photomicrograph showing a metallographic image of the sample of identification 1 described in Table 2 below. FIG. 2 is a drawing-substituting photomicrograph showing a metallographic image of the sample with identification 2 described in Table 2 below. FIG. 3 is a drawing-substituting photomicrograph showing a metallographic image of the sample with identification 3 described in Table 2 below. FIG. 4 is a drawing-substituting micrograph showing a metallographic image of the sample of identification 4 described in Table 2 below. FIG. 5 is a drawing-substituting micrograph showing a metallographic image of the sample with identification 5 described in Table 2 below. FIG. 6 is a drawing-substituting photomicrograph showing a metallographic image of the sample of identification 6 described in Table 2 below. FIG. 7 is a drawing-substituting photomicrograph showing a metallographic image of the sample of identification 7 described in Table 2 below. FIG. 8 is a diagram showing a change in YS × EL value according to time according to the low-temperature heat treatment temperature using the sample of the present steel type 1 described in Table 1 below. In the figure, the broken line indicates the case where the low-temperature heat treatment time is 15 minutes, the solid line is 60 minutes, and the alternate long and short dash line is 360 minutes. FIG. 9 is a diagram showing a change in YS × EL value for each temperature according to the low-temperature heat treatment time using the sample of steel type 1 of the present invention described in Table 1 below. In the figure, the broken line indicates the case where the low-temperature heat treatment temperature is 300 ° C., the solid line is 400 ° C., and the alternate long and short dash line is 500 ° C.

  Hereinafter, the present invention will be described based on embodiments. However, the present invention is not limited to these embodiments.

Examples of the present invention will be described below together with comparative examples that deviate from the conditions of the present invention.
Steel types 1 having the chemical composition according to the present invention and steel types 2 to 4 in which the Mo content deviates from the chemical composition according to the present invention were prepared. The chemical composition is shown in Table 1. Next, in the steel type 1 of the present invention shown in Table 1, steels having the metal structure of the present invention (identification 1 to 5) and steels having a metal structure deviating from the present invention (identifications 6 and 7) were produced. Table 2 shows the metal structures of these steels. Table 3 shows the production conditions of these steels. The hardness (HV), tensile strength (Ts), 0.2% proof stress (YS), and elongation (EL) of the manufactured steel (identifications 1 to 7) were measured and shown in Table 4, respectively. In Tables 1 to 4, numerical values with “*” on the left side indicate values that are outside the scope of the present invention.

[Table 1]

[Table 2]

[Table 3]

[Table 4]

As can be seen from the above results, according to the identifications 1 to 5 in Table 4, 0.2% yield strength (YS) exceeding 1400 N / mm ² is satisfied, and the γ phase exhibits elongation (EL) exceeding 15%. be able to. On the other hand, the identifications 6 and 7 as comparative examples cannot satisfy both of these characteristics at the same time. 1 to 7 show metallographic images of the samples with these identifications 1 to 7.

Next, steel type 2-4 having a composition deviating from the present invention was prepared together with steel type 1 having the composition according to the present invention shown in Table 1, and a stainless steel strip was manufactured based on various manufacturing conditions shown in Table 6. The metal structure is shown in Table 5, and the characteristics are shown in Table 7. In Tables 5 and 7, the symbol “*” written at the beginning of the numerical value means that the numerical value is outside the scope of the present invention.
The following can be understood from the experimental results shown in Tables 5-7. That is, in the steel type having the composition according to the present invention, if the low temperature heat treatment temperature does not exceed 500 ° C., the target characteristics of the present invention can be obtained regardless of the length of the low temperature heat treatment time. When the temperature is 500 ° C., the desired characteristics cannot be obtained if the low temperature treatment time is long. Moreover, the target characteristics cannot be obtained unless low-temperature heat treatment is performed.
On the other hand, with the steel type having a composition deviating from the present invention, even if the low temperature heat treatment is performed at the heat treatment temperature according to the present invention, the intended characteristics in the present invention cannot be obtained.

[Table 5]

[Table 6]

[Table 7]

FIG. 8 is a diagram showing a change in YS × EL value according to time according to the low-temperature heat treatment temperature when the process shown in Table 6 was carried out using a sample of steel type 1 of the present invention.
FIG. 8 shows that when the low-temperature heat treatment temperature exceeds 480 ° C., the target YS × EL value cannot be obtained especially when the low-temperature heat treatment time is lengthened. Conversely, when the low-temperature heat treatment temperature is less than 250 ° C., it can be seen that the desired YS × EL value cannot be obtained especially when the low-temperature heat treatment time is short. And if it is the range of 300 to 450 degreeC, it turns out that a desired YSxEL value can be stably obtained, without being dependent on the length of low-temperature heat processing time substantially.

FIG. 9 is a diagram showing a change in YS × EL value for each temperature according to the low-temperature heat treatment time when the process shown in Table 6 was performed using a sample of steel type 1 of the present invention.
FIG. 9 shows that the YS × EL value is stable at a low value of 22000 or higher at 300 ° C., and the YS × EL value is highly stable at a value of 29000 or higher at 400 ° C. On the other hand, at 500 ° C., the YS × EL value sharply decreases in the range of about 37000 to 20000 as the low-temperature heat treatment time becomes longer. From this, it can be seen that at a low temperature heat treatment temperature of 500 ° C. or higher, there is an inconvenience that abrupt degradation of characteristics occurs due to the low temperature heat treatment time, resulting in instability of quality.

In the present invention, the C content is 0.05 to 0.15%, the Si content is 0.05 to 1%, the Cr content and the Ni content are 16 to 20% and 4 to 11 respectively. %, Mo content of 2.5% to 3.5%, and further selected from the group of Al content of 0.1% to 3.5% and Ti content of 0.1% to 3.5% Or based on metastable austenitic stainless steels, including two. Then, the metastable austenitic stainless steel is subjected to low-temperature heat treatment preferably at 250 ° C. to 480 ° C. with a work induction martensite phase (α ′ phase) of 50% or more obtained by cold working as a parent phase. It is a two-phase structure of a processing-induced α ′ phase and a γ phase (γ _R phase + γ _T phase) obtained by performing, and the γ _T phase area ratio defined in the above formula 2 is 1% or more and 20% or less, the remainder of phase is stainless steel strip or steel sheet having a metallic structure consisting α'and gamma _R.
Such a low temperature heat treatment at 480 ° C. or less is a novel technique that does not reversely transform the metal structure of a general-purpose steel grade with Ni or Mn of 11% or less. Further, according to the above structure obtained by this method, The α ′ phase satisfies 0.2% proof stress (YS) exceeding 1400 N / mm ² , and the γ phase develops elongation (EL) exceeding 15%.
The metastable austenitic stainless steel used as the base has a high Cr and Ni content and has superior corrosion resistance compared to conventional iron-based high-strength, high-ductility steel sheets. Therefore, not only strength and workability but also corrosion resistance. It can also be expected to be used for applications that require the In addition to the above properties, a stainless steel strip or steel plate of HV450 or higher can be obtained depending on the application requiring hardness.

Claims (4)

In mass%, C: 0.05 to 0.15%, Si: 0.05 to 1%, Mn: 2% or less, Cr: 16 to 18%, Ni: 4 to 11%, Mo: 2.5% ~ 3.5% and Al: 0.1% to 3.5% and Ti 0.1% to 3.5%, one or two selected from the group, the balance being Fe and inevitable impurities Consists of
In 2-phase structure of α'-phase and gamma-phase, gamma-phase is composed of a gamma _T phase and the gamma _R phase, a total of 15 to 50% by volume of gamma _T phase and the gamma _R phase, defined by the following formula 2 The γ _T phase area ratio is 1% or more and 20% or less,
0.2% yield strength (YS) is in ^{1400N / mm 2 ~1900N / mm 2} , shown in the following formula 1 "YS-EL balance" is characterized by having a characteristic satisfying at least 21,000 to 48,000, metastable austenite Stainless steel strip or steel plate.
“YS-EL balance” = YS · EL Equation 1
γ _T- phase area ratio (%) = 100 × (total area ratio of γ _T- phase in the entire observation area) Equation 2
However, the α ′ phase is a work-induced martensite phase, the γ phase is a phase in which the γ _T phase and the γ _R phase are combined, and the γ _T phase is a reverse transformed austenite phase having an area per particle of 5 μm ² or more and 20 μm ² or less, The γ _R phase represents an austenite phase other than the γ _T phase, YS represents 0.2% yield strength, and EL represents elongation.   The metastable austenitic stainless steel strip or steel plate according to claim 1, which is HV450 or more. In mass%, C: 0.05 to 0.15%, Si: 0.05 to 1%, Mn: 2% or less, Cr: 16 to 18%, Ni: 4 to 11%, Mo: 2.5% ~ 3.5% and Al: 0.1% to 3.5% and Ti 0.1% to 3.5%, one or two selected from the group, the balance being Fe and inevitable impurities A step of preparing a stainless steel strip or steel plate comprising:
Cold-working the stainless steel strip or steel plate to form a work-induced martensite phase (α ′ phase) of 50 volume% or more from the austenite phase (γ phase);
The stainless steel strip or steel plate on which the work-induced martensite phase (α ′ phase) has been formed is subjected to low-temperature heat treatment in the range of 250 ° C. to 480 ° C., and the martensite phase (α A process for growing an austenite phase (γ _T phase) from a 'phase)), and having the following metal structure and mechanical properties, a method for producing a metastable austenitic stainless steel strip or steel plate.
In 2-phase structure of α'-phase and gamma-phase, gamma-phase is composed of a gamma _T phase and the gamma _R phase, the sum of gamma _T phase and the gamma _R-phase in 15 to 50% by volume, defined in Equation 2 γ _T phase area ratio is a metal structure of 1% or more and 20% or less,
0.2% yield strength (YS) is in ^{1400N / mm 2 ~1900N / mm 2} , shown in Formula 1 "YS-EL balance" has mechanical properties that meet the least 21000-48000.
“YS-EL balance” = YS · EL Equation 1
γ _T phase area ratio (%) = 100 × (total area ratio of γ _T phase in the entire observation area) Equation 2
However, the α ′ phase is a work-induced martensite phase, the γ phase is a phase in which the γ _T phase and the γ _R phase are combined, and the γ _T phase is a reverse transformed austenite phase having an area per particle of 5 μm ² or more and 20 μm ² or less, The γ _R phase represents an austenite phase other than the γ _T phase, YS represents 0.2% yield strength, and EL represents elongation.   The method for producing a metastable austenitic stainless steel strip or steel plate according to claim 3, wherein the stainless steel strip or steel plate is HV450 or higher.

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